It would take at least 20 years to adequately recapitalize Fannie Mae and Freddie Mac, a top Treasury official said Friday, the latest effort by the Obama administration to push back against arguments that the government-controlled mortgage-finance giants could be restored as private companies without legislation.

Mary Miller, the Treasury’s undersecretary for domestic finance, said in a speech in Washington on Friday that even if Fannie and Freddie were preserved and allowed to build up capital, that transition would take too long, maintaining a market that leaves taxpayers too exposed to losses and that isn’t serving the broad middle class.

While few in Washington expect the Senate to pass legislation this year, the administration has “not in any measure given up on housing-finance reform,” she said.

Fannie and Freddie have become very profitable over the last two years, but those profits are likely to fall this year, the companies have said, as various one-time benefits run their course. The mortgage companies have to shrink their large investment portfolios that have traditionally been a key profit engine.

As a result, “even if truly rehabilitating the [companies] were possible, recapitalizing them adequately would take at least 20 years,” Ms. Miller said.

The uncertain future of the companies has also contributed to the mortgage industry’s reluctance to extend loans to borrowers without perfect credit, she said, which has “effectively shut many Americans out of the [housing] market.” In opting for an administrative revamp of the companies, “we would also be signing up for another 20 years of under-serving responsible credit-worthy Americans seeking to buy a home,” she said.

Twenty years is a long time, and Ms. Miller didn’t address the assumptions behind that time frame, such as what would constitute an adequate level of capitalization. Many industry officials have said Fannie and Freddie would have withstood the latest crisis with at least 5% capital. The comments also hint at a long transition no matter what is done to resolve the limbo status of the companies.

Ms. Miller also defended a controversial arrangement in which the Treasury last year began collecting all of Fannie and Freddie’s profits as dividend payments, arguing that replacing an existing 10% dividend was necessary to assure markets that the companies wouldn’t need to borrow money to fund those dividend payments.

The changes were “all designed for the purpose of protecting the [companies] and making sure that we could keep the doors open,” she said. The Treasury Department is fighting around 20 lawsuits from investors challenging those changes.

Some industry executives have suggested that pieces of Fannie and Freddie, such as their apartment-financing units, could be sold off while the companies remain in a government-run conservatorship, but Ms. Miller said those ideas were a nonstarter with Treasury. “We would not consider any spin-off without legislation,” she said.

The Treasury on Thursday said that Ms. Miller would leave the administration in early September after more than four years at the department.

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